This invention relates to a light-emitting element matrix array, particularly to a light-emitting element matrix array in which a light up state of light-emitting element may be controlled by a small control current.
In an light-emitting element array used in an optical printer, it is essentially required to derive the same number of lines from light-emitting elements as that of light-emitting elements. A wire bonding method is generally used for the derivation of lines. Therefore, the following problems are caused as the density of light-emitting elements is increased.
(1) A product cost becomes larger due to the increase of an area of a wire bonding pad on a light-emitting element array chip, i.e. an area of a chip.
(2) A production cost becomes larger due to the increase of the number of wire bondings.
(3) A production becomes difficult as a pitch of wire bonding becomes smaller.
(4) A product cost is increased because driving circuits, the number thereof is equal to that of light-emitting elements, are generally required.
An area of one bonding pad is several times or more compared with an area of one light-emitting element, so that the increase of light-emitting elements density leads to the increase of chip area.
In order to avoid these problems, a light-emitting element including a shift register, a light-emitting diode (LED) matrix array, and a light-emitting thyristor matrix array have been proposed heretofore.
The LED matrix array as shown in FIG. 1 has resolved above-described problems by providing a plurality of light-emitting diodes in one line on an insulating substrate, and constituting a matrix with an anode side and a cathode side to decrease the number of terminals derived from the array.
In FIG. 1, one or more of light-emitting diodes L1, L2, L3, . . . may be lighted up by a combination of levels of anode electrodes A1-A3 and levels of cathode-selecting lines K1-K4. When an anode electrode Ai is at a High (H) level and a cathode selecting line Kj a Low (L) level, a light-emitting diode Lj+4(ixe2x88x921) is lighted up. However, a current flows from an anode driver (not shown) to a cathode driver (not shown) through a light-emitting diode, so that both of drivers require a large current driving capacity, resulting in the cost increase of driver ICs.
In order to resolve this problem, a light-emitting thyristor matrix array using a light-emitting thyristor of pnpn-structure in place of a light-emitting diode has been proposed. FIG. 2 shows the light-emitting thyristor matrix array. According to this matrix array, a plurality of light-emitting thyristors T1, T2, T3, . . . are arrayed in one line. These thyristors are divided into groups four by four. Anodes of thyristors in each group are commonly connected to anode terminals A1, A2, A3, . . . , respectively, gates of thyristors in each group are separately connected to gate-selecting lines G1-G4, and cathodes of all the thyristor are commonly connected to a cathode line K. The lighting up of thyristors T1, T2, T3, . . . is determined by a combination of levels of gate-selecting lines G1-G4 and levels of anode terminals A1, A2, A3, . . . . As this matrix array is a type of cathode common, when the cathode line K is at L level and an anode terminal Ai is at H level while one gate-selecting line Gj being at L level and the others H level, a light-emitting thyristor Tj+4(ixe2x88x921) is lighted up.
The gate-selecting lines only give trigger signals, so that the lighting up of the thyristors may be controlled by a small current driving capacity. As a result, the cost of a driving IC may be decreased. For this matrix array using a light-emitting thyristor, Japanese Patent has already been issued to the present applicant (Japanese Patent No. 2807910).
However, there is a problem such that the product cost thereof is high because such thyristor uses a pnpn-structure.
The object of the present invention is to provide a light-emitting element matrix array in which a function equivalent to a light-emitting thyristor may be implemented by elements different therefrom.
According to the present invention, a function of a light-emitting thyristor may be implemented by a combination of a transistor and light-emitting diode (LED). In this case, an LED may be fabricated by emitter and base layers or base and collector layers of a transistor, so that the combinational element of a transistor and an LED may be implemented from a three-layer structure of npn or pnp. Therefore, the thickness of an epitaxial film becomes thinner compared with a light-emitting thyristor of a four-layer pnpn-structure, resulting in a low cost of products.
Furthermore, a light-emitting element matrix array in which a light up state of light-emitting element may be controlled by a small control current may be implemented by integrating transistors and LEDs in the same wafer. Accordingly, the number of terminals derived from a light-emitting element matrix array chip may be decreased, thereby reducing an area of the chip. As a result, a high resolution light-emitting element matrix array may be easily provided.